1. Field of the Invention
The present invention relates generally to image sensors, and more particularly, to a CMOS (complementary metal oxide semiconductor) image sensor with a main photodiode having a diffusion layer thereon for minimizing dark current and with a pocket photodiode for minimizing image lag.
2. Background of the Invention
An image sensor is a semiconductor device that converts an image into electrical signals. An image sensor may be a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor (CIS). A unit pixel of a CMOS image sensor includes a transfer transistor that transmits charge generated by a photo sensitive device (PSD) to a floating diffusion region.
However, the CMOS image sensor has dark current generated therein from charge traps at dangling bonds of a hetero-interface such as an interface between a silicon substrate and a silicon oxide layer. As is well known, such dangling bonds are positioned between a valence band and a conduction band in an energy band and thus capture and shift electrons to the conduction band regardless of reception of light. Such shifted electrons are accumulated at the PSD even when an image is not captured, and consequently a color spot or a white spot appears on a screen.
In addition, when the transfer transistor transitions from being turned on to being turned off, a reverse electric field is formed between the transfer transistor and the PSD. Such reverse electric field causes a feed through effect in which some of the electrons flowing across the transfer transistor flow backward to the PSD. The feed through phenomenon causes an image lag with a ghost appearing on a reproduced image. 
The dark current may be traded-off for the image lag in a CMOS image sensor. Therefore, a CMOS image sensor with minimized dark current and image lag is desired.